Since the early 1950s,
crude oil has been seeping into the Colorado River, which is adjacent
to an oil field in Wharton County, Texas, U. S. A. The volume of oil discharging
to the river has changed over time; increases in discharge apparently
correlate with increased rainfall. River levels have also changed significantly
over time as a result of drought or flood conditions. Multiple seeps occur
over 182 m along the east bank of the river. Since 1936, oil-field features
in the area immediately adjacent to the east bank have included (1) numerous
oil wells, one of which blew out in 1959; (2) six surface tanks that contained
oil or saltwater; and (3) as many as four former earthen pits reported
to have contained crude oil, saltwater, or drilling mud.

In August 1999 we
assessed the possible sources of oil by collecting continuous cores and
installing groundwater-monitoring wells at 12 locations. Strata are deltaic
clays interspersed with thin channel sand deposits of the Beaumont Formation.
The clay is highly fractured most likely as a result of rapid removal
of overlying sediments when the river was established in its present day
location. During drought conditions the fractures in the clay that has
a high shrink-swell potential are most likely open.

The measured plan-view
hydraulic gradient is toward the river. We found hydrocarbon impacts (elevated
concentrations of ionizable compounds and TPH) in six of the cores as
deep as 14 m and free-phase product as thick as 0.4 m in two monitoring
wells downgradient of the largest earthen pit and upgradient of the seeps.
We hypothesize that oil from the pit migrated downward through fractures
in the clay, collected at the interface with the underlying water-bearing
sand at a depth of ~15-m, and is moving laterally toward the river.